DESCRIPTION: IgA antibodies provide an important first line of defense against mucosal pathogens. The induction of immunoglobulins against T cell-dependent antigens is subordinate to T cell activation, T/B cell interaction and cytokines. Our long-range goal is to learn how the mucosal immune system responds to protein antigens of the virulent and commensal microbiota. Our current objective is to determine, in vivo, which type of T cell response is induced by the commensal Lactobacillus murinus, while transiting the intestine. We hypothesize that intestinal commensal microorganisms transmucosally induce a T cell-dependent humoral response, while suppressing the cell-mediated response. Testing this hypothesis can lead to the design of effective microbial delivery systems. We will test this hypothesis by determining 1) how a commensal microbe primes naive T cells in the intestine and 2) the type of cytokines elicited in memory T cells. We engineered L. murinus to expresses an ovalbumin epitope that induces ovalbumin-specific T cell proliferation when injected subcutaneously. We inject a small population of ovalbumin-specific T cells into recipient mice and feed L. murinus in high numbers. Ovalbumin-specific T cells are tracked by staining cell suspensions or tissues of the recipients with anti-CD4 and an anti-T cell receptor monoclonal antibody. Four-color flow cytometry and confocal immunohistology will establish in vivo that commensal microorganisms transiting the intestine activate T cells to induce CD69 expression. In an antigen-specific manner we will test the phenotype and kinetics of transmucosal T cell activation, proliferation and differentiation into cytokine-producing memory cells. Collectively, this research will elucidate the missing link between the oral antigen delivery and the production of secretory antibodies. The data will be important to human health, establishing a framework to study in vivo mucosal infectious agents and oral vaccines.